Whereas the clinical and prognostic relevance of cell cycle kinetic properties of neoplastic cells was a subject embroiled in controversy in the past, the relatively recent introduction of monoclonal antibodies to thymidine analogues such as bromodeoxyuridine (BrdU) and iododeoxyuridine (IUdR) has dramatically altered our perceptions. It is now not only possible to characterize large numbers of patients with myeloid disorders for detailed cell cycle kinetics, but given their impressive prognostic value and the fact that data are available in a prompt fashion, this information can provide the basis for prospective manipulations of the biologic properties of leukemia cells. The goal of this project is to explore the phenomena of proliferation in patients with acute myeloid leukemia (AML). New methods have been developed to investigate the cell cycle kinetics in biopsies using two DNA specific probes i.e., Iudr and BrdU in vivo. The effects of chemotherapy as well as cytokines can also be measured by the sequential administration of these agents. These incisive studies will surely lead to an improve understanding of biology and are already forming the basis for the design of our future therapeutic protocols in these disorders.